One of the fundamental issues in high power AC switching applications is the large arc or flashover generated when making or breaking any switching contacts on load. This arc is caused by the switch attempting to open or close at any random part of the AC sine wave cycle and therefore inadvertently switching the full load current.
The undesirable effects of switching at random points in the AC cycle are made worse if the circuit being switched exhibits a poor power factor, that is, either the AC source or the AC load contain reactive elements.
Due to its somewhat explosive nature and high temperature, this arc can cause significant erosion of the switching contacts which, at best, can shorten the contact life or in extreme circumstances, can weld the contacts permanently closed. Both of these are considered serious switch failures. Ideally, for maximum lifetime, the switch should always operate at the zero crossing point in the sine cycle. However, this is not always possible due to delay or latency between the switch contacts reacting to a control signal in automated switching and the random nature of manual switching (e.g. inserting a switching link). Conventional techniques employed by power switch manufacturers to reduce arcing include the use of mechanical bellows to draw the arc away from the switch contacts and the use of inert gases to extinguish the arc at source.
US 2008/0048807 discloses an arrangement with an AC relay and a triac connected in parallel to one another, the triac being turned on just before and turned off just after the contacts of the AC relay are opened or closed. The triac is turned on and off at the zero crossings of the AC to minimise power surges, and aims to prevent arcing across the mechanical contacts of the AC relay.
The minimisation of power surges is important to help protect load components, and to prevent excessive current from flowing under faulty load conditions, for example a short-circuit. Loads typically comprise some form of protection circuitry such as a fuse at the input, although an excessive level of current may still flow during the time before the fuse blows, and the source power supply may still be adversely affected.